North Korea has ramped up the rhetoric against South Korea again through its weapon of choice this year: the fax machine.

South Korea’s Defense Ministry said Friday a letter from the North’s National Defense Commission addressed to the South’s presidential office was faxed early Thursday via the military communication link between the two sides, threatening a “merciless” attack on South Korea.

The letter objected to the “repeated extra-large provocations to North Korea’s highest dignity taking place in the middle of Seoul” and warned of “a merciless retaliation without warning,” according to ministry spokesman Kim Min-seok.

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The threat was a reference to demonstrations held in the South by conservative activists and North Korean refugee groups this week to mark the second anniversary of the death of Kim Jong Il.

The ministry faxed a response back that promised “resolute punishment” would follow any provocation from the North, Mr. Kim said. He added that there weren’t any unusual signs in the North’s military activity, though annual winter drills are taking place.

Associated Press

A fax machine.

Pyongyang’s fax tactics came into play earlier this year when South Korean firms that run factories in the jointly-run Kaesong Industrial Complex inside the North received faxes blaming Seoul for the plants’ prolonged closure.

South Korea said at the time the faxed letters were a ploy to turn public opinion against the Seoul government.

Mr. Kim declined to provide further details of the fax threat or the history of fax exchanges with Pyongyang.

Other than faxes, the two Koreas have other channels of communication. Daily phone calls are made at the border to coordinate traffic into the Kaesong complex, although North Korea pulled the plug on phone links during the escalation of tensions this spring.

Physical documents are exchanged at border-town of Panmunjom, where the 1953 armistice was signed after the Korean War. The Kaesong plant also has an administrative office where civilian officials from the two Koreas speak to each other, a spokeswoman at Seoul’s Ministry of Unification said.

She confirmed there is no e-mail communication between the sides.

A more unconventional method comes in the form of leaflet flights, with South Korean activists sending information about the outside world–and condemnation of North Korea’s regime–northward in helium-filled balloons.

North Korea has sent leaflets to the South, though their delivery methods aren’t clear. Leaflets threatening attack on a South Korean border island were found on the island this week, according to local media reports.

The tables have been turned against the North’s fax machines before. The Voice of the Martyrs, an Oklahoma-based Christian activist group that fights church persecution worldwide, said in 2009 it sent messages about the outside world and bible passages to North Korean fax numbers for about a year.

In June that year, the organization said it received a response – through fax – saying “something very bad will happen” if the efforts continued, according to the group’s website.